1. Technical Field of the Invention
The present invention relates to a sheet transport unit for separating and conveying a sheet from stacked sheets one by one by a conveying mechanism and for transporting the sheet downstream by a feed roller, and a recorder provided with the sheet transport unit.
2. Description of the Related Art
A transport unit, such as a sheet transport unit used in an ink jet printer has been known, which separates and conveys a sheet from the top of stacked sheets of paper by a conveying (or separating) roller coming into contact with the top of the stacked sheets and, then, feeds the separated sheet in front of a recording head or the like by a feed roller. The conveying roller has an outer circumference section capable of contacting with the sheet on the top of stacked sheets to feed it, and a notch section receded from the outer circumference section. In such transport unit, when a leading edge of the sheet reaches the feed roller and the feed roller starts to feed the sheet, since the notch section of the conveying roller faces a trailing portion of the sheet, the power transmission from the conveying roller to the sheet is interrupted. However, rotatable collars are provided on a coaxial axis with the conveying roller, the distance between the sheet and the conveying roller is held by the collars.
In another type of transport unit, the conveying roller is made to be able to rotate freely after the interruption of power transmission, thereby the conveying roller can support the trailing edge of the sheet which has pulled out by the feed roller.
However, even if the collars or the conveying roller rotates freely, as its resistance is not null, when the trailing edge of a sheet of paper exits from the collars or the conveying roller (hereinafter, conveying roller and collar are collectively called "conveying mechanism"), the resistance exerting on the sheet reduces suddenly, affecting the feed amount by the feed roller.
In other words, while the sheet is subjected to the resistance from the collars or the conveying roller, the feed roller receives a force moving upstream in its feeding direction as a reaction to the sheet transportation. In general, a clearance exists necessarily between the shaft and the bearing of a feed roller and such backlash displaces the feed roller upstream in the feeding direction. When the trailing edge of the sheet exits from the conveying mechanism, its reaction force reduces suddenly, and the feed roller advances downstream in the feeding direction in the clearance, affecting the feed amount. Particularly, when the recorder proceeds to record the next line, after having recorded one line, a line feeding by the feeding roller is performed. If the feed amount of this line feeding is affected, it occurs a problem that the recorded image is disturbed in the image recorder. In the ink jet printer, a line feeding discrepancy of several tens micrometer(.mu.m) badly affects the printing performance.